Fix typo

Closes gh-1364

spring-context/src/main/java/org/springframework/validation/Validator.java

@@ -55,7 +55,7 @@ package org.springframework.validation;
  * }</pre>
  *
  * <p>See also the Spring reference manual for a fuller discussion of
- * the {@code Validator} interface and it's role in an enterprise
+ * the {@code Validator} interface and its role in an enterprise
  * application.
  *
  * @author Rod Johnson